Solid state light sources such as light emitting diodes (LEDs) are often preferred light sources for many applications because of their small size, pure colors, and long lifetime. However, packaged LEDs are not often optimized for any particular application and designers go to lengths to collect and direct light output for a particular use. Light that is not efficiently utilized is effectively wasted.
Optically transmissive materials, such as glass or polymers may be used as a light guide to propagate light. A light guide often includes at least one surface adapted to receive light from a light source and an optically smooth surface for reflecting light propagating through the light guide. Common examples of light guides include optical fibers traditionally used in the data communication industry and more recently light guides used for illumination purposes. For example, U.S. Pat. No. 5,432,876 discloses one such illumination device employing light guides. In this device, light may be injected into at least one end of a light guide and allowed to exit the guide at a predetermined position or positions along the length of the guide. Light extraction structures or notches are formed in the guide. The extraction structures define first and second reflecting surfaces, which reflect in a radial direction a portion of the light propagating axially through the guide. The reflected light is directed at an angle that is less than the critical angle necessary for continued propagation along the guide according to the principle of total internal reflection. As a result, the reflected light is extracted from the guide.
Collecting light from a lambertian LED source and coupling that light into a light guide is often difficult since this light emitted in all directions from the LED and this light must be redirected into the numerical aperture of the light guide where it can be efficiently maintained by total internal reflection. When a simple reflector is used, the imperfect specular reflectivity of the reflection surface means that some portion of the light is lost on every reflection bounce. A reflector that is designed for collimation of a lambertian light source usually requires a large aspect ratio to achieve the desired output angle. This aspect ratio also means a relatively large number of bounces for light traversing the reflector, and more bounces equate to more loss.